I wonder if some noticed there is one rule that both BG3 and Solasta changed and I have seen NO ONE complain about it !
In both games, you can cast an Action spell and a Bonus Action spell in the same turn (ie. I do NOT mean a cantrip, but rather you can use two spell slots/turn in both games).
In vanilla 5e, you can use one single spell slot (spell level 1+) along with one single cantrip in one turn. Meaning you cannot use a spell slot for an Action and then another spell slot for a Bonus Action in a single turn. Example : In one turn, you could cast Firebolt *cantrip* (Action) and Healing Word (Bonus Action).[note 1] The rule is not bypassed by having an extra Action (the limit is per turn. If you get an extra turn with some poweful magic, then sure, you can cast more spells).
Anyhow, this flexibility in BG3/Solasta seems to please most people as no one ever mentions that homebrew !
The germane text from the player's handbook: "A spell cast with a bonus action is especially swift. You must use a bonus action on your turn to cast the spell, provided that you haven't already taken a bonus action this turn. You can't cast another spell during the same turn, except for a cantrip with a casting time of 1 action." This doesn't speak at all to a limit on the number of spell slots you can use per turn.
The rule is bypassed with Action Surge (as mentioned in the tweet that you linked, in fact) - you can cast two full-action spells in one turn with Action Surge so long as you aren't also using your bonus action to cast a spell (because that would trigger the rule).